1. Field of the Invention
The present invention relates generally to the art of permeability testers, and more particularly to improved apparatus for measuring the permeability of sheet material to passage of a fluid.
2. Description of the Prior Art
The permeability of a material relates to the ability of a fluid, usually a liquid or gas, to pass through the material. Some materials are impervious, that is, unable to accommodate fluid penetration; others are pervious to some fluids, but impervious to others. There are many uses for pervious, semipervious, and impervious materials.
Surgical instruments are commonly stored in sterile packages. One face of the package is usually a transparent plastic; the other face may be a treated paper product. After the instrument has been sealed in the package, the entire package may be subjected to a suitable sterlizing gas, such as ethylene oxide. The plastic material is typically impervious to passage of both air and ethylene oxide. However, it is desired that the paper be pervious to ethylene oxide, but impervious to bacteria.
Devices for measuring the permeability of a material to a particular fluid have, of course, been heretofore developed. In one test, which appears to have been widely adopted in the industry, the permeability of a paper is measured in terms of the length of time needed for a volume of 100 cc of air, supplied at a pressure of 4.88 inches of water, to pass through 1.0 in.sup.2 of the paper. Such paper is then commonly referred to as "twenty second paper", "twenty-seven second paper", and so forth. The industry-wide standards for such tests are currently embodied in ASTM Standard D726-58, entitled "Standard Methods of Test for Resistance of Paper to Passage of Air".
One device for measuring permeability is shown in Bulletin 1400, entitled "Paper Testing Instruments", by Teledyne Gurley, 514 Fulton St., Troy, N.Y. 12181. That brochure described the test apparatus as clamping 1.0 in.sup.2 of paper, and supplying 100 cc of fluid (air) at a pressure of 4.88 inches of water. The air is supplied by a weighted cylinder floating in oil, and the volume is determined by the change in cylinder position. The "start" and "stop" points of such cylinder position may be measured electrically, as by two contact points, or photo-electrically. Upon study, however, it is felt that this apparatus provides numerous opportunities for human error, and therefore may be unreliable from a practical point of view. Moreover, it is believed that oil from the volumetric container may contaminate the test sample and invalidate the test.